Brillouin/erbium fiber laser current monitor using elliptically polarizing fiber

ABSTRACT

A laser arrangement comprising a ring cavity at least two gain portions in said ring cavity, an input-output coupling connected to said ring cavity, a discriminator means connected in said ring cavity and adapted to discriminate between a pumping signal and an output signal and pumping means for providing said pumping signal and connected to said gain portions of said ring cavity such that, upon activation of said pumping means, said laser arrangement operates as a laser producing a laser output at said input-output coupling.

FIELD OF THE INVENTION

The present invention relates to a Brillouin/Erbium laser arrangementand in particular, to a bidirectional single elliptical polarizationhigh efficiency and output power all fibre laser arrangement and itsapplication to current sensing.

BACKGROUND OF THE INVENTION

Brillouin fibre lasers (BFL) are particularly useful in fibre sensorsdue to extremely narrow linewidths and they have been the subject ofconsiderable research at a number of different wavelengths [S. P. Smith,F. Zarinetchi, and S. Ezekiel, Opt. Lett., vol. 16, p. 393,1991]. Theyhave found applications such as gyroscopes [S. Huang, K. Toyama, B. Y.Kim, and H. J. Shaw, Opt, Lett., vol. 18, pp. 555-557, 1993] and currentsensors [A. Küng, P. -A. Nicati, and P. A. Robert, IEEE Photon. Technol,Lett., Vol. 8, p. 1680, 1996]. Such devices are realized by using twocounter-propagating Brillouin lasers sharing the same fibre ringresonator and experiencing different apparent cavity lengths due tononreciprocal phaseshifts induced by either Sagnac or Faraday effects.These two counter-propagating Brillouin lasers will therefore oscillateat slightly different optical frequencies and the optical beat frequencywill be linearly proportional to either the rotation rate or theelectric current. These types of sensors have much simpler signalprocessing than more conventional devices [I. G. Clarke, Opt. Lett.,vol. 18, pp. 158-160, 1993] based on Sagnac loop. To achieve maximumsensitivity of the current sensor to the Faraday effect it is necessaryto keep the laser polarization states circular although some degree ofellipticity is acceptable.

The conventional technique to produce a BFL is to construct a criticallycoupled fibre resonator, required because of the small magnitude of theBrillouin gain [G. P. Agrawal, in Nonlinear Fibre Optics, AcademicPress, San Diego, Calif., 1989]. Principal disadvantages of criticallycoupled BFLs include the small output power that can be achieved, therequirement of cavity matching to the pump signal, and the difficulty inincorporating intra-cavity elements because of their associated loss.

Brillouin/erbium fibre lasers (BEFL) [G. J. Cowle and D. Yu. Stepanov,J. Lightwave Technol., VOL. 15, PP. 1198-1204, 1997] combine Brillouingain as used in BFLs with gain from erbium-doped fibre (EDF) to yieldlaser sources with unique properties. Narrow bandwidth nonlinear gainfrom stimulated Brillouin scattering in Single Mode Optical Fibre (SMOF)precisely determines the wavelength of operation, and gain inerbium-doped fibre (EDF) allows efficient operation and large powerextraction. BEFLs do not require cavity matching to the Brillouin pumpfrequency since the BEFL resonator is not critically coupled. TheBrillouin pump can be removed from the laser cavity to avoid spuriousinjection locking. In [G. J. Cowle and D. Yu. Stepanov, Opt. Lett., vol.21, pp. 1250-1252, 1996] the laser cavity was made non-resonant in thedirection of the Brillouin pump injection using a pigtailed bulkisolator. However, such an approach is undesirable for the sensingapplications which require bidirectional operation.

SUMMARY OF THE INVENTION

It is an object of at least preferred embodiments of the presentinvention to provide an improved laser arrangement which overcomes thedisadvantages of the aforementioned laser arrangements and is furtherpreferably capable of being applied to electric current measurements.

In accordance with a first aspect of the present invention there isprovided a laser arrangement comprising a ring cavity, at least two gainportions in said ring cavity, an input-output coupling connected to saidring cavity, a discriminator means connected in said ring cavity adaptedto discriminate between a pumping signal and an output signal, andpumping means for providing said pumping signal and connected to saidgain portions of said ring cavity such that, upon activation of saidpumping means, said laser arrangement operates as a bidirectional laserproducing a laser output at said input-output coupling.

In one embodiment, the laser arrangement comprises a ring cavity withBrillouin gain portions, a bidirectional optical amplifier and anelliptical polarizer interposed in predetermined portions of the ringcavity, input-output couplings connected to the ring cavity, and pumpingmeans connected to the Brillouin gain portions of the ring cavity, suchthat, upon activation of the pumping means, the laser arrangementoperates as a bidirectional laser and produces an output at saidinput-output couplings.

Preferably the laser arrangement acts as a Brillouin laser for obtaininga bidirectional single polarization high power output at the frequenciesshifted down from the frequencies of the pumping means by an amountdetermined by the Brillouin shift in the glass fibre.

In accordance with the second aspect of the present invention theaforementioned laser arrangement is applied to the measurement of anelectric current by measuring the optical beat frequency betweencounterpropagating waves in the ring cavity. The optical beat frequencybeing linearly proportional to the current value.

Preferably, the optical amplifier is formed from the one piece of rareearth doped optical fibre with a separate pumping means connected to therare earth doped optical fibre.

Preferably, the elliptical polarizer is formed from the one piece ofspun highly birefringent polarizing fibre (PSHBF) polarizing in thewavelength range of the laser operation [I. G. Clarke, Opt. Lett., vol.18, pp 158-160, 1993].

The Brillouin pump should be removed from the ring cavity to enable thelaser to be operated in the BEFL mode. This can be achieved using anumber of different techniques with the preferred embodiment utilising apolarizing spun highly birefringent fibre. Alternatively, a blazedgrating or other discriminators, could be utilized, provided that thediscriminator does not inhibit the bidirectional operation of the laserarrangement.

In broad terms, the preferred embodiment of the present inventionprovides a Brillouin laser, comprising Brillouin gain portions and arare-earth doped optical amplifier inserted into a ring cavity.

More specifically, the preferred embodiment of the present inventionprovides a bidirectional single polarization high output power laserarrangement at the frequencies shifted down from the frequencies of thepumping means by an amount determined by the Brillouin shift in theglass fibre, comprising Brillouin gain portions for obtaining gain inopposite directions at shifted frequencies, input/output couplers toprovide input/output coupling and feedback through the ring cavity, anoptical amplifier to compensate for the ring cavity losses, and apolarizing spun highly birefringent fibre (PSHBF) to prevent injectionof the Brillouin pumping means into the optical amplifier.

The concept of the laser arrangement is as follows: First increase thegain of the optical amplifier to a level just below that required forthe arrangement to operate as a laser. Elliptically polarized light froman external narrow linewidth laser is then injected into thepredetermined portions of the ring cavity to stimulate Brillouinscattering in both clockwise and counterclockwise directions. Inscalar-type backward stimulated Brillouin scattering, circularly orelliptically polarized Brillouin pump is reflected into the Stokes waveas if from a conventional mirror [B. Ya. Zeldovich, N. E. Pilipetsky,and V. V. Shkunov, in Principles of Phase Conjugation, Springer-Verlag,Berlin, 1985], ie. with the reversal of rotation sense and preservationof the orientation of the ellipse. With sufficient intensity of theinjected Brillouin pump an elliptically polarized Brillouin signal isgenerated in the opposite direction and with the opposite rotation sensesupported by the PSHBF at a frequency separated from the injected signalby the Stokes shift and it will eventually have sufficient gain to reacha lasing threshold by virtue of the combination of the Brillouin gainand the gain in the optical amplifier.

Preferable key features of the laser are that it does not need a lowloss resonator, instead using an optical amplifier to equalise theresonator losses, which allows it to obtain high power output, and itdoes not require an optical isolator, instead using a PSHBF to isolatethe Brillouin pump, thus allowing for the bidirectional operation. ThePSHBF prevents both depletion of the optical amplifier gain by aninjection locking to the Brillouin pumping means and provides feedbackfor the Brillouin signal.

In a preferred embodiment, the Brillouin gain portions are lengths ofsingle mode optical fibre and the optical amplifier comprises a sectionof erbium doped fibre which is optically pumped through a wavelengthdivision multiplexing (WDM) coupler. With sufficient pump power applied,the ring laser would operate as a normal erbium doped fibre ring laser.However, in this embodiment, laser action is originated from signalsshifted down in frequency by an amount determined by the Brillouin shiftin the glass fibre, generated from an external narrow linewidth lasersource. The Brillouin pump signals injected in opposite directions arenon-resonant in the laser cavity as they are blocked by the PSHBF in thering and hence do not enter the EDF section of the ring. However theBrillouin signals generated in the opposite directions with respect tothe corresponding Brillouin pumps are amplified by the amplifyingsection of the ring. The polarizing spun highly birefringent fibreprevents the ring from operating as an injection locked laser and setsthe internal polarization state of the laser.

Such an arrangement has advantages in producing bidirectional highoutput power narrow linewidth laser sources for fibre sensors. The laserhas only one elliptically polarized eigenstate in both clockwise andcounter-clockwise directions. This makes the laser very attractive forcurrent sensing using the Faraday effect. Additionally, the laser alsohas application in the gyroscope area. Elliptical polarization of thecounter propagating laser eigenstates also ensures that spatial holeburning in the amplifying section of the laser is reduced as thestanding wave pattern is close to a helicoidal shape and thus the lightfield intensity approaches uniformity along the doped fibre.

To measure nonreciprocal phaseshifts induced by the Faraday effect whenan electric current is applied to a conducting coil wound around thePSHBF section of the laser, the frequency of the optical beat producedby the counter-propagating lasing modes can be analysed using aradio-frequency spectrum analyser (RFSA). For experimental convenienceand to prevent possible lock-in of the counter-propagating lasing modesat low currents, the optical beat frequency in this embodiment ispreferably biased using a frequency shifter by shifting the frequency ofone of the Brillouin pumps, thus shifting the corresponding Brillouingain maximum. Given the sweep time τ of the RFSA is much longer than theelectric current waveform period T and the ratio τ/T is known, thewaveform can be reconstructed from the RFSA spectrum and the current canbe measured.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following description of embodiments thereof, by wayof example only with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a laser arrangement constructed inaccordance with an embodiment of the present invention;

FIG. 2 shows the optical spectrum of mixed Brillouin pump andBrillouin/erbium fibre laser signals. The Brillouin pump is shown fortwo orthogonal polarization states, with one polarization state blockedby the PSHBF;

FIG. 3 illustrates a typical optical beat frequency spectrum measuredusing an RFSA;

FIG. 4 illustrates an AC current waveform reconstructed from the opticalbeat frequency spectrum; and

FIG. 5 presents the measured value of the optical beat frequency widthvs the mean value of the electric current.

DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

FIG. 1 schematically illustrates a Brillouin/Erbium fibre laserarrangement 1 which includes lengths of single mode-optical fibre 2 and3, a section of erbium doped fibre 4, and a polarizing spun highlybirefringent fibre 5. The arrangement also comprises input couplers 6and 7 to provide input coupling and to complete the ring. A wavelengthdivision multiplexing (WDM) fibre coupler (not shown) is coupled to andprovides pumping of the section of erbium doped fibre 4.

In a first example, the resonator was formed by a fused fibre couplerwhich couples 50% of light out of the ring, a 5 m length of Er/Al doped(250 ppm) fibre, WDM coupler to allow pumping of the doped fibre with980 nm light from a pigtailed semiconductor laser, two 50 m sections ofsingle mode optical fibre 2, 3 and a 50 m polarizing spun highlybirefringent fibre 5. A copper wire 16 with 1.5 cm² cross section waswound around the PSHBF 5 and an AC electric current was applied to thewire.

The laser was pumped with a 980-nm pump laser diode through a 980/1550WDM located in the ring. Brillouin pumps were injected into the lasercavity through the input couplers 6 and 7 in opposite directions, withone pump frequency shifted by 27 MHz using an acousto-optic frequencyshifter 8. This allowed the application of a bias to the optical beatfrequency. A tunable semiconductor laser diode with a 0.001 nm settingaccuracy was used as a Brillouin pump which was split into two channelsusing a splitter 9. Its wavelength was tuned close to the wavelength ofspurious lasing in the absence of the Brillouin pump.

FIG. 2 illustrates a graph of the optical spectrum of the resultantlaser output mixed with the optical spectra of the two orthogonalpolarizations of the Brillouin pumps adjusted using polarizationcontrollers 10 and 11 at the input couplers 6 and 7 to provide eitherminimum or maximum transmission through the PSHBF 5. The optical spectrawere observed using taps 12, 13, a mixer 14 subsequently connected to anoptical spectrum analyser (OSA). The Brillouin signals were Stokesshifted by about 0.08 nm from the Brillouin pump wavelengths and theoperation of the laser was observed to be very robust. The curve 17measures the optical spectra of the BEFL output with the curve 18plotting maximized transmission and the curve 19 plotting the minimizedtransmission through the PSHBF of the residual Brillouin pumps. Thecounterpropagating signals are not distinguishable in the opticalspectra as the frequency separation of 27 MHz falls far below theresolution of about 10 GHz of the OSA employed. From the measurements ofthe Brillouin pump transmission, the extinction ratio of the polarizingspun highly birefringent fibre was estimated to be better than 45 dB.

The arrangement 1 of FIG. 1 has a potential tunability over a certainwavelength range by tuning the Brillouin pump wavelengths within therange of spurious lasing.

FIG. 3 illustrates a typical optical beat frequency spectrum of the BEFLoutput when a 50 Hz AC electric current was applied to the copper wirecoil 16. The spectrum was measured using an RFSA. The sweep time of theRFSA trace was 10 s. The spike-like shape of the spectrum 20 is causedby the stroboscopic effect as the sweep time of the RFSA in thisparticular case was an even number of the electric current periods.

Since during the measurements both the AC current and the measured beatfrequency are functions of time it is possible to reconstruct the ACwaveform and the result of this reconstruction is shown in FIG. 4.Further, the amplitude of the waveform vs mean value of the electriccurrent applied is plotted in FIG. 5 which clearly illustrates itslinear nature.

It will be further appreciated that the present invention is not limitedto an all fibre arrangement and that other elliptical polarizers andring cavities may be employed.

It will be further appreciated that the present invention is not limitedto an elliptically polarizing spun highly birefringent fibre and thatother polarizing fibres may be employed

It will be further appreciated that other signal processing techniquescan be employed.

It will be further appreciated that other sensor applications of thelaser arrangement such as gyroscopes can be utilized.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiment without departing from the spirit or scope ofthe invention as broadly described. The present embodiment is,therefore, to be considered in all respects as illustrative and notrestrictive.

We claim:
 1. A laser arrangement comprising: a ring cavity; at least twogain portions in said ring cavity; an input-output coupling connected tosaid ring cavity; a discriminator means connected in said ring cavityand adapted to discriminate between a pumping signal and an outputsignal; and pumping means for providing said pumping signal andconnected to said gain portions of said ring cavity such that, uponactivation of said pumping means, said laser arrangement operates as abidirectional laser producing a laser output at said input-outputcoupling.
 2. A laser arrangement as claimed in claim 1 wherein saiddiscriminator means comprises an elliptical polarizer.
 3. A laserarrangement as claimed in claim 2 wherein said ring cavity and saidelliptical polarizer comprise optical fibre.
 4. A laser arrangement asclaimed in claim 3 wherein said elliptical polarizer comprises apolarising spun highly birefringent fibre.
 5. A laser arrangement asclaimed in claim 2 where said elliptical polarizer acts as a filter toremove Brillouin pumping means from the laser cavity and to provide asingle polarization output.
 6. A laser arrangement as claimed in claim 1wherein one of the gain portions comprises a rare-earth doped fibre. 7.A laser arrangement as claimed in claim 6 wherein said rare earth iserbium.
 8. A laser arrangement as claimed in claim 1 wherein one of thegain portions comprises a single mode optical fibre (SMOF).
 9. A laserarrangement as claimed in claim 8 wherein the pumping means connected tothe SMOF gain sections comprises a narrow linewidth laser source andacts as Brillouin pumps for signals propagating in counter directions.10. A laser arrangement as claimed in claim 1 wherein said laserarrangement is applied to electric current or spatial rotationmeasurements.